In the construction of semiconductor circuits, a large wafer is made first. To construct the individual circuits, the wafer is cut into smaller pieces called dies or chips. Each finished individual circuit is composed of a single die sealed in a two part package. Wires attached to the terminals on the die extend out of the package. In order to ground the die and to hold it in place in the package, the die is bonded to the package base.
At present, at least two different methods are used to bond dies to a package base. The first method is a eutectic gold silicon alloy, which must be performed under high heat. The gold silicon alloy has several drawbacks. First, because the bond must be made under heat, there is a great risk that the heat will oxidize the package and die. Second, to obtain 100% eutectic coverage under the die requires near perfect flatness of the die and package. Third, the gold silicon alloy and the die have different coefficients of expansion than the package. When the chip is heated or cooled, by use or by the environment, the gold silicon alloy and the package expand or contract at a different rate, which stresses the die. The stress can affect the operation of the circuitry in the die. If exposed to enough stress, the die can shatter.
Another material used to bond the die to the package is a silver glass paste. This paste is applied at a lower temperature and after firing has a coefficient of expansion which is very similar to that of the die and the package, avoiding the stress problems of the gold silicon alloy. However, with the silver glass paste, it is very important that the thickness of the paste in the bond, i.e., the distance between the package and die, be uniform across the entire mating surfaces of the package and die. It is also important that the overall thickness of the paste in the bond be within certain parameters.
The silver glass paste is a conductor, but not a perfect conductor. If the paste is too thick, i.e., the die is too far from the package, the resistance between the die and package is too high. As a result, the die will not be fully grounded, and excessive resistance heating develops in the paste. The heat negatively affects the circuitry in the die. If the paste is too thin, i.e., the die is too close to the package, then the package and die will not be sufficiently isolated and the bond strength will be negatively affected. Also, excess paste squeezed from between the package and die can lap over the top of the die and cause shorts. Similarly, if the paste thickness is not consistent across the width of the die, then hot spots can develop where the silver glass paste is thicker. Uneven heat stresses the die and affects the die's circuitry inconsistently. Gaps in the paste prevent the die from being fully grounded.
In the past, it has proven difficult to achieve uniform and consistent bond thickness with existing techniques. A typical die bonder uses a pneumatic clamp to lift individual dies by their top surface and a holding fixture to hold the package base by its bottom surface. The silicon glass paste is then placed on the package, and the die is pressed into the paste. An excess amount of paste is applied to the package, so that when the die is pushed into the paste, the excess paste is squeezed from between the die and the package. The squeezing helps ensure that there are no gaps or voids in the paste bond.
Two methods are commonly used to control the bond thickness. One method is to press the die into the package with a predetermined force. This method assumes a uniform silver glass paste viscosity, so that a consistent force results in a consistent bond thickness. However, the silver glass paste viscosity is not easily controlled, making the results generally inconsistent.
Another method depresses the die into the paste a preset distance. The preset distance is determined by the distance from the clamp to the holding fixture. This method assumes that the thickness of the package and the thickness of the die are consistent. However, since the desired bond thickness is on the order of 0.001 inch, variations in the thickness of the die and the package can result in substantial variations in the bond thickness.